Six United States marines are missing after an accident over the coast of Japan early on Thursday morning, Reuters reported. Officials suspect that two US Marine Corps aircraft may have collided mid-air during a refuelling operation and then crashed into the Pacific Ocean.

Japan’s Defence Ministry said it had rescued one person so far and efforts were on to bring the others to safety. The person was on the F/A-18 Hornet fighter jet, and their condition had now stabilised.

“The incident is regrettable, but our focus at the moment is on search and rescue,” Japanese Defence Minister Takeshi Iwaya said. “Japan will respond appropriately once the details of the incident are uncovered.”

The Marine Corps said the incident occurred around 2 am on Thursday, about 322 kilometres off the Japanese coast, and that there was a “mishap” during regular training of the F/A-18 and the KC-130 Hercules refuelling aircraft.

“We are thankful for the Japan Maritime Self-Defense Force’s efforts as they immediately responded in the search and rescue operation,” the Marine Corps said in a statement. Officials said an investigation has begun.